Keep an eye on the sky for spring and fall bat migrations.

03.30.24

By Paul Hormick

western small-footed myotis, J. Scott Altenbach

When it comes to migration, birds get all the glory. Canada geese fill the skies as they migrate through North America. A parade celebrates the migration of cliff swallows as they arrive in Capistrano, California each spring, and their journey from South America has even been lauded in popular song

Hidden by the cover of darkness, few of us see migrating bats, let alone know much about their journeys. That’s where science steps in.  

Like other migrating animals, bats migrate for two things: warmer weather and food. During winter, bat food is scarce or nonexistent. Flowers don’t bloom, fruits have fallen from trees, and insect numbers plummet. For warm-blooded animals like bats, they expend energy just to stay warm during the colder months of the year. 

Some bats wait out winter by hibernating in warm, damp places like caves, taking on a state of torpor for up to six months. Other bats are forced to migrate. Possibly because they are more exposed to the elements, most tree roosting species, such as the hoary bat, migrate south to Mexico for winter. Scientists studying the big brown bat found they use the Earth’s magnetic field to orient themselves during migration, calibrating their biological compasses with the sun. 

Mexican long-nosed bat at agave flower, Horizonline Pictures.

Migration, particularly in warmer climates, can be variable. Depending on food availability, bats may migrate one year and stay put the next. Some bats have a “follow the flowers” migration pattern. The Mexican long-tongued bat lives in Central America and Mexico, but migrates north to feed on the flowers of blooming agaves and columnar cacti in Arizona, New Mexico, and Texas. Its large body size enables it to store energy and travel over areas where little of its food can be found. The lesser long-nosed bat, has a similar distribution, but can migrate 1,000 miles in pursuit of nectar and fruit from saguaro and organ pipe cactus in the early summer months, followed by agave nectar later in the summer in Arizona and New Mexico.  

While most bats have a shorter migrations than birds, some bats are travelling long distances. A Nathusius’s pipistrelle recently made bat headlines by flying from Russia to the French Alps, a distance of 1,500 miles. And Australia’s flying foxes were thought to have a small migration pattern, moving about 100 miles between “camps” and following the location and concentration of flowers. But more recent research finds that they are actually more nomadic, travelling over 3,000 miles a year, with some individuals trekking even farther.

The Greatest Migration 

Bat viewing sites, international
Straw colored fruit bats take to the air at Kasanka National Park, Zambia, Dr. Brett Lewis at Kasanka Trust.

In a surprising twist, one of the largest mammal migrations in the world is that of a bat species. From October through December, eight million or more straw-colored fruit bats gather at an evergreen swamp forest in Zambia’s Kasanka National Park. If measured in weight, that number of bats would tip the scales at 3,500 tons. Befitting the mysterious nature of bats, this event remained pretty much unknown until recent years. 

Believed to migrate from somewhere in Congo, the bats descend on Kasanka to feed on pod mahogany and other wild fruit that come with the first rains. Listed as a near threatened species by the International Union for Conservation of Nature, efforts are being made to protect the bat. They are now seen as essential for the ecology of large areas of central Africa, as they spread millions of seeds during their migration. 

Challenges Humans Present to Migrating Bats

Wind turbines, Michael Schirmacher

Wind turbines kill hundreds of thousands of bats each year in the United States. The danger increases for birds and bats when they migrate, with bats suffering far more fatalities than birds. Moving the turbines offshore was initially thought to reduce this risk, however ongoing research and historical records indicate that bats are common over the open ocean, leading scientists to think that offshore turbines have an unknown level of risk to bats.

Climate change is also affecting bat migrations. Mexican free-tailed bats, which make the yearly trek from Mexico to Bracken Cave outside San Antonio, Texas, are arriving earlier than they did 20 years ago, most likely due to warmer temperatures. The bats serve as natural pest control, feasting on insects that consume farm crops. These bats certainly need to eat after their long journey, but if the bugs haven’t hatched when the bats arrive, the bats could go hungry. Fewer bats will lead to more insects consuming food crops. 

Using climate change modeling, scientists predict that warming temperatures could reduce the overlap between the migration range of the Mexican long-nosed bat and the agave plants they pollinate by more than 75 percent. This could lead to the bat’s extinction and threaten the diversity of agave as well. Other species are experiencing similar migration hazards. 

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